The generation of small devices often involves an alignment of multiple material layers, e.g., two or more material layers, with the assistance of an imaging system. For example, in one scenario, the material layers are positioned substantially parallel to each other and are spaced apart by an amount that is relatively small compared to their overall dimensions. An imaging system having an optical axis perpendicular to the material layers is used to image one or more alignment markers on or within each layer. Relative position is established according to alignment marker positions. Alignment markers can comprise markers provided explicitly for alignment purposes, or can comprise known landmarks identified among features present on the material layer.
One issue can arise in the context of material layers that can obscure other material layers relative to the imaging system. As used herein, a first layer obscures a second layer if a clean image of the second layer cannot be obtained by the imaging system in the presence of the first layer. Depending on the selection of algorithm used for alignment sensing or control, the presence of obscuring material layers may confound performance in the alignment sensing and/or control process. Notably, a first layer may obscure a second layer even if the first layer is positioned below the second layer relative to the imaging system. For example, features or textures of a lower layer may show through an upper layer such that a clean image of upper layer cannot be obtained in the presence of the lower layer.
Other issues that can arise, either alone or in combination with obscuring material layers, relate to the ever-shrinking nature of devices and the ever-changing nature of their fabrication processes. One issue brought about by shrinking feature sizes is that the spacing between material layers may begin to exceed the depth of field of the imaging system used in the alignment process. Another issue is that it may not always be desirable or practical to use alignment markers for sensing device alignment and/or for sensing relative position shifts. At the same time, however, the need for exacting alignment tolerances remains. By way of example, it may be desirable to measure very small changes in an alignment state between layers even where no alignment markers are present.